Methods of bonding semiconductor elements and lead frames (supporting members) during production of semiconductor devices include methods wherein a filler such as silver powder is dispersed in a resin such as an epoxy-based resin or polyimide-based resin to form a paste (for example, silver paste), which is used as a bonding agent.
According to this method, a dispenser or a printer or stamping machine is used to coat the paste-like bonding agent on the die pad of a lead frame, and then the semiconductor element is subjected to die bonding and heat cured for bonding to produce a semiconductor device.
The semiconductor device is mounted by soldering on a circuit board after the exterior has been sealed with a sealing material for semiconductor packaging. Because recent mounting requires high density and high efficiency, solder mounting is accomplished primarily by surface mounting methods wherein the lead frame of a semiconductor device is directly soldered to a board.
Such surface mounting employs reflow soldering whereby the entire board is heated with infrared rays, and the package is heated to a high temperature of 200° C. or above. When moisture is present during this time inside the package, and especially in the adhesive layer, the moisture becomes gasified and surrounds the die pad and sealing material, resulting in generation of cracks (reflow cracks) in the package.
Such reflow cracks significantly reduce the reliability of the semiconductor device and thus constitute a serious problem and technical issue, and therefore bonding agents widely employed for bonding of semiconductor elements and semiconductor supporting members are required to have reliability, including bonding strength at high temperature.
Furthermore, with the increasing speeds and higher integration of semiconductor elements in recent years, demand has also risen for high heat dissipation properties to ensure the operating stability of semiconductor devices, in addition to the conventional requirements of reliability including bonding strength. That is, efforts to find solutions to these technical problems have led to demand for connecting materials with both high bonding strength and a high coefficient of thermal conductivity, for use as bonding agents for bonding of heat-dissipating members (lead frames) and semiconductor elements.
In addition, one proposed means for achieving higher heat dissipation than conductive adhesives, which utilize contact between conventional metallic particles, has been the use of conductive adhesives that employ metal nanoparticles with mean particle sizes of up to 0.1 μm, which have excellent sintering properties, or wherein metal fine particles are sintered at high temperatures of 200° C. or above. This prior art is described in Patent documents 1-5, for example.
The conventional method for ensuring a high coefficient of thermal conductivity of bonding agents involves high filling of silver particles with a high coefficient of thermal conductivity. It has also been attempted to increase heat conduction and ensure strength at room temperature by using low melting point metals to form heat conduction paths by metallic bonding and metallize adherends. Conductive adhesives employing metal nanoparticles are also being studied.